# How to determine Economic Transmission Voltage?

The transmission voltage for which the cost of conductors, transformers, insulators, switchgears and other terminal apparatus is minimum is called the economical transmission voltage.

As we know, if the transmission voltage is increased, the volume of conductor material required for the transmission lines is reduced. Hence, this decreases the expenditure on the conductor material. It is advisable to use the highest possible transmission voltage in order to reduce the expenditure on the conductor material to a minimum. Although, it may be remembered that as the transmission voltage is increased the cost of insulation, transformers, switchgears and other terminal equipment also increases. Therefore, for every transmission line, there is an optimum transmission voltage, beyond which there is nothing to be gained in the matter of economy.

## Methods of Determining Economical Transmission Voltage

In order to determine the economical transmission voltage, the power to be transmitted, generation voltage and length of the transmission line are assumed to be known.

In this method, we select some standard transmission voltage and work out the following costs −

• Conductor – The cost of conductor material decreases with the increase in transmission voltage.

• Transformers – For a given amount of power, the cost of transformers, used at the generating and receiving ends of the transmission line, increases slowly with the increase in transmission voltage.

• Insulation and Supports – The cost of insulation of conductors and supports (like poles & towers) increases sharply with the increase in transmission voltage.

• Switchgear – The cost of switchgear also increases with the increase in transmission voltage.

• Lightening Arrestor – The cost of lightening arrestor increases rapidly with the increase in transmission voltage.

The sum of all above costs gives the total cost of transmission of electric power for the voltage considered. Similar calculations are made for other transmission voltages. Then, a graph is plotted between the total cost of transmission and the transmission voltage as shown in the figure. Here, the lowest point (M) on the curve gives the value of economical transmission voltage. This method of determining the economical transmission voltage is rarely used in practice because the above given costs cannot be determined accurately.

In practice, an empirical formula is used for finding the economical transmission voltage. The formula used for determining the economical transmission voltage between lines in a three-phase AC system is given as

$$\mathrm{\mathit{V}\mathrm{\, =\, }5.5\times \sqrt{0.62\mathit{l}\mathrm{\, +\, }\frac{3\mathit{p}}{150}}}$$

Where,

• V is the line voltage in kV.

• l is the length of transmission line in km.

• P is the per phase maximum power in kW.

From the above formula, it can be observed that the power to be transmitted and the length of the transmission line have been taken into account because both these factors affects economic voltage of transmission line.